Spinal fixation devices may be surgically implanted in the body to effect a desired relationship between adjacent vertebral bodies. Such devices typically include a rigid stabilizing rod coupled to one or more devices for anchoring the rod to the vertebral bodies. The stabilizing rod must be contoured to accommodate variations in patient anatomy as well as variations in desired therapeutic benefits. Since each vertebral body varies in size and shape, a variety of anchoring devices have been developed. Pedicle screws have a shape and size appropriate for engaging pedicle bone. Using implantable multi-axial pedicle fixation systems known in the art, surgeons may be challenged to obtain optimal bone purchase while obtaining optimal stabilizing rod position.
A need exists for an implantable multi-axial pedicle fixation system with a primary coaxial drive feature that maximizes driver interface while minimizing the bone fixator geometry required to withstand functional loading, thus increasing the degree of total multi-axial angulation available, and with a secondary non-coaxial drive feature that transfers the torsional loads required to advance or retract the bone fixator from the vertebral pedicle, lateral mass, or transverse process.